SIP ALG is controlled per device attached to the router, it’s not a setting for the router as a whole.

1. Log into your router
2. Navigate to: Settings>Advanced Settings>Business Network>Devices
3. Select a device by clicking on it
4. Change ‘Disable SIP ALG’ to ‘Yes’
5. Hit ‘Apply’

This applies to the BT Business Hub 3 (BH3) and possibly also the Home Hub 3 (HH3).